Update sanitize.sh

[DaveRead]

Errors from expr sometimes occur if environment variables are parsed which contain parentheses.

Instead of testing for non-zero length keys, the test for identifying keys to inspect can use the key length, only checking those that are at least 3 characters long (e.g. might start with BM_).

For example, this environment variable creates the issue:

BASH_FUNC_which%%=() {  ( alias;
 eval ${which_declare} ) | /usr/bin/which --tty-only --read-alias --read-functions --show-tilde --show-dot "$@"
}

By checking that the parsed key length is greater than 2, unexpected parsing of the environment is less likely to create spurious error messages.